Stem cells are cells that have the potential to develop into different cell types in the body during early life and growth. They have the ability to self-renew and are integral in the body's natural repair process. There are two primary sources of stem cells, embryonic and non-embryonic or adult stem cells. Adult stem cells are found in practically every tissue or organ in the body. They too have the ability to self-renew and differentiate into a multitude of specialized cell types.
Mesenchymal stem cells (MSCs) are a specific group of mesoderm origin adult stem cells that are pluripotent. Being pluripotent, they have multi-directional differentiation capabilities. They can become fat, bone, cartilage, tendons, muscle, nerves, ligaments, liver, cardiac muscle, endothelial cells, pancreatic islet cells and many others. In addition, they are cells with low immunogenicity and are naturally immune-modulatory cells. Given their versatility, MSCs have quickly become an ideal cell type used in therapeutics for degenerative and autoimmune conditions.
MSCs have the unique ability to navigate or home to areas of injury and/or degeneration. When the body is in need of repair it sends out signals to mobilize the stem cells to begin the repair process. MSCs not only differentiate but increase angiogenesis and excrete anti-inflammatory cytokines and growth factors. MSCs were originally found in bone marrow. It was soon discovered that in elderly or ill adults, the MSC content in bone marrow is extremely low. Low stem cell yield and a painful donation process led scientists to look for other, more easily available sources of MSCs in the body. This search led them to fat tissue.
Adipose tissue contains approximately 100,000 MSCs per gram of fat (Sen et al., 2001). It is a naturally rich source of MSCs and they are mostly unaffected by age or the donor's condition. Fat is becoming very popular as a stem cell source because of its ease in extraction and in most cases, ample availability.
Having a large amount of fat tissue may translate into a high stem cell count but acquiring a large amount is a fairly invasive procedure. Liposuction often requires general anesthesia and vacuum suction. When machine suction is used, cells are often broken and injured during the extraction process. Therefore, small, localized syringe extractions are ideal. In this case, a 5 gram extraction would yield approximately 500,000 MSCs. To reach therapeutic quantities of MSCs (in the millions or billions), in vitro cell culture is a typical solution.
Culturing fat derived MSCs is much easier compared to other sources. They generally proliferate well and behave consistently regardless of the donor's age or condition. However, their proliferative potential and their stem cell characteristics are continuously decreased during prolonged culture. For example, it has been shown that expansion in culture leads to premature senescence (the process of aging characterized by continuous morphological and functional changes). Cells became much larger with irregular and flat shape and the cytoplasm became more granular. These senescence-associated effects are continuously acquired from the onset of in vitro culture (Wagner et al., 2008). As a result, the successful manufacturing for commercialization of large batches from one donor of homogenous MSCs that preserve their characteristics following expansion in culture remains a challenge.
Methods for increasing proliferation and survival in MSCs have been widely studied over the past few years and many factors have been proposed for increasing the expansion efficiency of these cells. For example, many protocols relating to the expansion of MSCs include culturing in the presence of basic fibroblast growth factor (b-FGF) (Colleoni et al., 2009). It has been shown that b-FGF not only maintains MSC proliferation potential, it also retains osteogenic, adipogenic and chondrogenic differentiation potentials through the early mitogenic cycles. Vascular endothelial growth factor (VEGF) has also been shown to increase MSC proliferation (Pons et al., 2008). Hepatocyte growth factor (HGF) has been shown to affect proliferation, migration and differentiation (Furge et al., 2000). Platelet derived growth factor (PDGF) shown to be a potent mitogen of MSCs (Kang et al., 2005). Epidermal growth factor (EGF) and heparin-binding EGF have both been shown to promote ex vivo expansion of MSCs without triggering differentiation into any specific lineage (Tamama et al., 2006; Krampera et al., 2005). In addition to its mitogenic effect on MSCs, EGF also increases the number of colony-forming units by 25% (Tamama et al., 2010).